Chair back



FIG. 1 is a perspective view of a chair back, showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is a left side view thereof;

FIG. 5 is a right side view thereof;

FIG. 6 is a top plan view thereof; and,

FIG. 7 is a bottom plan view thereof.

The broken lines depict portions of the chair back in which the design is embodied that form no part of the claimed design. 

CLAIM The ornamental design for a chair back, as shown and described. 